Elements of the tobacco plant are known to comprise a proportion of lipids, a component of varying composition, variously defined but most commonly referenced for convenience as included within the hydrocarbon solvent extractables, e.g. from petroleum ether or hexane.
Recently lipids have occassioned interest in the continuing analysis of health related aspects of the use of tobacco. Specifically, it has been suggested that the hydrocarbon solvent solubles contribute to the generation of polynuclear aromatic hydrocarbons during leaf pyrolysis. Experimental results demonstrate that about 70% of the aromatic hydrocarbons, ranging from benzene to benz(.alpha.)pyrene in the pyrolysates are due to leaf components extractable with hexane and acetone, although these extracts amount to less than 25% of dry leaf weight. See "Studies on the Pyrogenesis of Tobacco Smoke Constituents: A Review" Chortyk, O. T. and Schlotzhauer, W. S. Beitrage zur Tabakforschung (Vol. 7, #3 Nov. 1973, pp. 165-177).
There are numbers of lipid fractions characterized by the extraction technique employed. The neutral lipids (commonly defined by high solubility in chloroform) form the major portion of tobacco leaf lipid material, comprising about 60-75% of the total. Accordingly, the most common lipid extraction techniques are based upon chloroform. Other solvent extraction methods are intended principally to remove tobacco solubles and nicotines. In either case, solvent extraction inevitably removes additional chemical constituents, often those which contribute favorably to taste and aroma. Accordingly, methods are desirably identified for selective treatment of tobacco to minimize the lipid fraction while retaining the physical and chemical integrity of the remaining components.
The neutral lipids which may be observed on tobacco leaf surfaces and are generally associated with the glandular trichomes (hairs) which cover the leaf surface and the cuticle layer just beneath these hairs, may be removed by mechanical techniques, as more fully disclosed in copending and commonly assigned application Ser. No. 556,025, filed Mar. 6, 1975 now U.S. Pat. No. 4,018,234.
The internal lipids are generally of somewhat different composition, and may be differentiated into further fractions believed to interrelate with composition.
Some efforts have been made to isolate such fractions in the handling of defatted soybean flakes, and certain solvent treatments are reported to improve color and flavor and to modify the properties of soy proteins. See AIChE Symposium Series:Food Preservation, Vol. 69, (1973) No. 132 pp. 5-9, and references cited. However, odor and flavor observations developed by direct application of food compositions to the tongue or palate are not comparable to the smoking experience involving a complex of aesthetic and olfactory responses to the spectrum of individual components developed in mainstream and sidestream smoke in the course of pyrolysis, pyrosynthesis and distillation.
Indeed, the difficulty with selective tobacco extraction may be traceable to the multiplicity of components involved: an extensive composition study on the hexane soluble material of flue-cured leaves (Chemistry and Industry, Vol. 14 1961 pp. 435-6) concluded that the bulk of the constituents could not be fractionated despite intensive effort.
Most importantly, little correlation is available to individually interrelate smokestream components to original tobacco treatment steps, ranging from chloroform or petroleum ether treatments to water washes, bleaches or even the complete extraction reported with a sequence of 95% ethanol, alcoholbenzene, 1% HCl, 5% sulfuric acid and water. Moreover, many such extraction processes are disclosed only for analytic purposes, and remove components exhaustively which are necessary to satisfactory taste in smoking.
Co-pending and commonly assigned application Ser. No. 556,080, Filed Mar. 6, 1975 now U.S. Pat. No. 4,144,895 describes the utilization of a particularized solvent extraction technique providing a substantial reduction in tobacco lipids including so-called bound internal lipids with minimized loss of solubles including nicotine and taste and aroma constituents. This result is found to ensue despite the use of a polar solvent component known to be especially effective in the removal of nicotine and tobacco solubles.
U.S. Pat. No. 2,822,306 describes the extraction of nicotine from tobacco with organic solvents containing limited amounts of water, the function of the water being for the purpose of substantially avoiding the co-extraction of oils, waxes, and resin, and at the same time favoring the removal of nicotine.
Mouron et al., in "Extraction of Tobacco by some Organic Solvents and Consequences on Chemical Composition of the Smoke" Oncologia 13, No. 2 (1960) pp. 128-135 refers to tobacco extractions with ethanol, in which system water does not exceed five percent, as being a bad solvent, and emphasizes halogenated hydrocarbon solvents in other testing of humidified tobacco.